1. Field of the Invention
This invention relates to a modular two-way check valve construction for delivering precise volumes of a liquid repeatedly. More particularly, this invention relates to a check valve construction which changes direction of fluid flow automatically.
2. Description of Prior Art
A variety of industries require the delivery of precise volumes of a liquid to an area of use. For example, precise volumes of a photoresist composition are delivered to a silicon wafer substrate in order to form a photoresist layer of uniform thickness onto the wafer. The requirement for precise volume delivery must be effected by an apparatus which is capable of delivering the same volumes of samples repeatedly over long periods.
A wide variety of apparatus utilizing a pump and a check valve arrangement presently are available for delivering liquids. The check valve is intersposed between a reservoir for the liquid and the point of use of the liquid and functions to control liquid flow from the reservoir in response to pressure on the liquid provided by a pump. Typically, a three way valve is utilized which is operated by a motor that responds to a signal from an electrical controller. The use of this arrangement is undesirable since the three way valve has a tendency to seize, and the arrangement requires a belt transmission and a motor which are expensive and are subject to breakdown.
Accordingly, it would be desirable to provide a two-way check valve apparatus capable of delivering precise volumes of liquid repeatedly. In addition, it would be desirable to provide such a check valve apparatus which is capable of changing fluid direction automatically while delivering precise volumes of liquid over extended time periods without the need for an auxiliary motor controlling the check valve apparatus.
The present invention provides a check valve construction which utilizes either two free floating plungers, each positioned within a separate liquid flow path or one free floating plunger and a spring loaded plunger, each positioned within a separate liquid flow path. A free floating plunger is positioned with a liquid flow path between a liquid reservoir and a pump. A second free floating plunger or spring loaded plunger is positioned within a liquid flow path between the pump and a point of use of the liquid. The free floating plunger and the spring loaded plunger function to permit liquid flow or to prevent liquid flow in response to a pressure generated by the pump. The free floating plunging and spring loader plunger are sized so that they do not restrict liquid flow when open. The check valve of the invention eliminates the need for a motor and accompanying motor support means for operating the check valve.